The present invention relates to a bar code reader and a bar code reading method for reading a bar code by irradiating the bar code with light beams and detecting a change in intensity of reflected light beams, and demodulating the bar code into data.
As typified by a POS (Point of Sales) system in a distribution sector over the recent years, it has been generalized that the merchandises are managed by bar codes. For instance, according to the POS system in a shop, pieces of data such as categories and sales prices of the goods are coded in a bar code format, and printed or pasted onto the goods. Then, the bar code of the goods is read at a cash register in a payment location, whereby the number of goods sold is added up in real time as well as doing the payment of a charge. This serves for a stock management and a purchase management.
By the way, the bar code described above is roughly classified into a fixed length code such as a WPC code, UPC code, an EAN code and a JAN code, and a variable length second code. This variable length code is categorized as a code in which a length (the number of bars) is not fixed in terms of standard. For example, an ITF (Interleaved Two of Five) code, CODE 39, NW 7 and CODE 128 may be exemplified. The fixed length code has such a structure that guard bars (GB) are added to both of side ends, a center bar (CB) is inserted in an intermediate area therebetween, and pieces of character data for 4 or 6 characters are interposed between each guard bar and the center bar. In this structure, even when only the first half character data are read, this set of character data are interposed between the guard bar (LGB) at the left side end and the center bar, and can be therefore recognized as the data constituting the bar code. Similarly, even when only the second half character data are read, this set of character data are interposed between the guard bar (RGB) at the right side end and the center bar, and can be therefore recognized as the data constituting the bar code. Accordingly, the read data of the whole bar code can be synthesized based on the data obtained by separately reading the first half data characters and the second half data characters.
A bar code reader for reading the thus structured bar code is roughly classified into a fixed type reader used for comparatively small goods formed with the bar codes, and a handy type reader used for comparatively large goods. The fixed type bar code reader thereof is provided with a mechanism for scanning the bar code in a multiplicity of directions by irradiating the bar code with light beams for reading so that an operator is capable of reading the bar code without being so aware of a bar code direction. More specifically, the bar code is scanned in the multiplicity of directions with the laser beams defined as reading irradiation light beams during a scan by one reflection surface of a polygon mirror used in combination with fixed reflection mirrors. Then, during such a scan process, the laser beams reflected from the surface of the goods are received, and a demodulation processing circuit demodulates change-of-intensity data (reflected light data) of the thus received reflected laser beams with a demodulation algorithm for the bar code and extracts data coded into the bar code.
The fixed type bar code reader described above has been requested to be more sophisticated with a high performance so that it is capable of precisely reading the bar code even when the goods passes therethrough at a high speed (a high moving speed with respect to the reader). For attaining this, there is made an attempt of enhancing a probability (a bar code scanning probability) at which the laser beams hit the bar code of the goods passing above the bar code reader by increasing a laser scanning speed with high-speed rotations of the polygon mirror. It is required for performing the high-speed scan described above that the number of rotations of a driving motor for the polygon mirror be increased. With this increase in the number of rotations of the motor, however, an electric current consumed might increase, which needs a power supply device having a large current capacity. Besides, a binarizing circuit is required to be mounted with a high-performance IC capable of responding to a high-frequency characteristic of an analog system circuit. These requirements might be factors for large increases in the car code reader itself and related equipment as well. Accordingly, it is a present situation that the great majority of bar code readers do not adopt the high-speed scanning method explained above. As a result, there arises a problem inherent in the conventional bar code reader, wherein the number of reading processes over the bar code is not sufficient, and the data of the bar code can be read approximately only once at the maximum if the goods passes therethrough at a high speed.
Further, in the prior art bar code reader, a slice level (a threshold value) for outputting white/black edge signals (WEG, BEG) when over a predetermined voltage, is provided as a criterion for judging whether black-and-white binarization is executed or not. This slice level is set comparatively high in order to prevent noises caused by ruggedness on the surface of the goods from being outputted as edge signals due to an influence by circuit noises.
In the thus constructed bar code reader, in the case of a bar coder with a low PCS value (a difference between black and white reflectance ratios) and a colored bar code, and if a passing distance between a bar code reading unit of the reader and the goods is large, a return light quantity of the reflected light decreases. With this decrease, an amplitude of a black-white changing waveform becomes small enough to be under the slice level, and consequently this might be judged as a nose, with the result that the edge signal is not outputted. Furthermore, in an area where the amplitude is small even if slightly over the slice level, an S/N ratio is poor, and the amplitude is under the slice level due to the influence by the circuit noises etc. This might lead to a case where the edge signal is not outputted.
Furthermore, if conditions for inducing the problems described above are overlapped, an amplitude of a differential signal a photoelectric conversion waveform corresponding to the bar code decreases under the slice level, with the result that plural lengths of bars appear as one length of bar. This makes the bar code data unable to be precisely extracted and demodulated.